In my efforts to learn as much as possible about the Arthurian Mythos, I have collected quite a few books on the subject. I feel it would only be good to tell others what medieval texts are currently in print, or are available online.

Welsh

The Four Ancient Books of Wales

Black Book of Carmarthen

Book of Aneurin

Book of Taliesin

Red Book of Hergest

The Mabinogion: print. various, but look for the Guest version, which includes the Hanes Taliesin.